1. Field of the Invention
The present invention relates broadly to a particular type of polyester shrink film. More specifically, this invention relates to a heat-shrinkable polyethylene terephthalate film coated with a solvent-based heat-seal coating and/or laminated to other films, useful in packaging, e.g., as bags or lidding stock.
2. Discussion of the Related Art
The use of heat-shrinkable thermoplastic films is well-known to the packaging industry. For example, poultry products are typically sealed within bags made from such films, and heated, thus shrinking the bag until it fits tightly around the product. One such bag is monolayer polyester film. These bags provide strength and protection through tight adhesion to the product, though they have the drawback that they must be sealed with an adhesive since polyester is not heat-sealable except at exceptionally high temperatures. Bags sealed with adhesive are generally not as strong in the seal area as heat-sealed bags, and cannot be closed on the open end by existing heat-seal equipment.
Coextruded films, such as polyolefins, are useful in producing heat-shrinkable bags because they are heat-sealable and therefore can be produced on existing heat-seal equipment economically. They maintain good physical contact with a packaged product after heat shrinking, and thereby retain juices within packaged meats, but not as well as laminated shrink bags. However, coextruded film have have less desirable mechanical properties, such as tensile strength and modulus, and therefore bags from these films are more apt to tear or otherwise become physically damaged during handling. As well, they generally do not possess high temperature heat resistance, which limits their application for cook-in uses.
U.S. Pat. No. 4,971,845 discloses an oriented heat-sealable, heat-shrinkable adhesive laminated film comprising, in one instance, a polyester film layer and a polyolefin film layer, wherein said film layers comprise similar shrink characteristics. The one example discussed combines layers each having xe2x80x9capproximately the same shrinking characteristicsxe2x80x9d, i.e., a shrinkage of about 50%.
There exists a need for a thermoplastic film which has high-strength, is heat-shrinkable and heat-sealable, as well as high temperature heat resistant.
This invention provides a heat shrink film for packaging comprising:
(a) a heat shrink film comprising a polymer having at least 80% by weight polyethylene terephthalate polymer, wherein said film is biaxially oriented in the range of about 5% to about 55%, said film having an outer surface and an inner surface; and
(b) a solvent-based, heat-seal coating on at least one of the outer surface and inner surface.
This invention also provides a method to package an article, comprising the steps of:
(a) wrapping the article in a shrink film comprising a polymer having at least 80% by weight polyethylene terephthalate polymer, wherein said film is biaxially oriented in the range of about 5% to about 55%, said film having an outer surface and an inner surface, said inner surface having coated thereon a solvent-based, heat-seal coating;
(b) heat-sealing the required sides to form a bag;
(c) sealing the bag with the article therein; and
(d) shrinking the bag by applying heat.
This invention further provides a tamper-resistant packaging container capable of venting vapor comprising, in combination:
a) an open rigid or semi-rigid container having a floor portion and side walls upwardly extending therefrom, said side walls having an outside surface;
b) a heat-shrink film having an inner surface and a solvent-based heat seal coating applied on such inner surface, said inner surface of said film in intimate sealing contact with at least a portion of the outside surface of the side walls, and wherein the heat-shrink film comprises:
i) a polymer having at least 80% by weight polyethylene terephthalate polymer; and
ii) wherein said film is biaxially oriented in the range of about 5% to about 30%.
This invention includes a tamper-resistant heat-shrink lidding film, having an inner surface, for covering a rigid or semi-rigid container having an outside surface, said film comprising at least 80 percent by weight of a polyethylene terephthalate homopolymer or copolymer, having a plurality of sides and a thickness of 12-75 micrometers, and being biaxially oriented in the range of about 5%-30% shrink factor; and said inner surface comprising a solvent-based heat-seal coating thereon, wherein:
a) said film is intimate contact with at least a portion of the outside surface of said container;
b) when the sides of said film are heated, said film remains in tamper resistant engagement with said container, hermetically sealing said container, yet allowing air and moisture to move in and out of said lidding film upon heating.
This invention also provides an improved method for laminating PET film to other films, comprising the steps of:
(a) coating a solvent-based laminating adhesive to the PET film;
(b) heating to less than the shrink temperature to dry the adhesive;
(c) laminating the film to other films without requiring additional adhesive.